This invention relates to the field of aircraft or projectile attitude determination and estimation using a GPS receiver; the system using a first and second linear accelerometer for an estimation of the projectile""s roll angle, and information from an additional third accelerometer for a pitch angle estimation.
The problem to be solved is how to guide a gun-fired projectile onto a target with a known geographic location at lowest cost. This application addresses a portion of that problem in providing an estimate of the trajectory of the projectile from launch, the initial conditions of the projectile immediately after launch and the distance, direction and altitude to the idealized trajectory calculated for the projectile at the time of launch or firing.
A first approach to the problem to be solved used only a GPS receiver and a turns counter. Such a system would be the least expensive approach but it would be vulnerable to GPS jamming. Additional funds would have to be expended on protecting the GPS signal.
A second approach used a GPS receiver, a turns counter, and a triax of gyros, which adds to the cost, and requires protection of the GPS signal almost all the way into the target. The protection is required because the gyros cannot account for any external forces which act on the projectile once the GPS signal is lost.
A third approach accounts for these external forces by adding accelerometers into the system such that the sensor package is flying a complete IMU. This package is expected is be too costly because of the gyros already in the package. Philosophically, one can either protect the GPS signal or operate with limited GPS data by requiring higher performance inertial instruments. The cost and robustness of providing GPS anti-jamming protection depends on the approach and the number, location and quality of the jammers. The GPS signal can be protected with anti-jamming electronics, and multi-beam antennae but at higher cost and complexity.
Gyroscopic instruments for aircraft use are well known and available in a number of technologies such as iron rotor and tuned rotor gyros, ring laser gyros, multi-oscillator gyros, zero lock gyros (ZLG), fiber optic gyros, resonator gyros such as HRGs or hemispherical or tubular ceramic resonant gyros and the like. Each of these systems have failure modes and uncertainties relating to a launch acceleration in the range of 15,000-30,000 Gs. Each of the technologies mentioned require that the vehicle carry at least one gyro in a gimbaled or strap-down arrangement with the attendant disadvantages of cost, weight and power dissipation.
A first alternative embodiment of the invention system and process uses a GPS and a triax of accelerometers with no gyros. An algorithm uses GPS position and delta GPS delta velocity data along with data from the accelerometer triax to solve for the projectile""s estimated attitude in pitch, roll and yaw. With the projectile""s position known from data provided by the GPS receiver, the algorithm calculates the projectile""s attitude in navigational coordinates and creates a time indexed record of the projectile""s trajectory after the on-board GPS receiver locks on to the required number of satellites. The data in the time indexed record of the trajectory is filtered and smoothed.
As the projectile rolls, the accelerometers are used to measure the forces acting on the projectile and the projectile""s rotation rates. The ultimate cost of this approach will depend on the required delivery accuracy which will drive the cost of the accelerometers that are to be used.
In some alternative embodiments of the invention, the GPS is used to provide data for the calculation of initial aiming and velocity errors and for the calibration of the accelerometers when positioned in the gun barrel prior to launch. Where cost must be reduced, much lower accuracy accelerometers can be used with a greater reliance on GPS signal data after launch.